<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        *{margin: 0;
        padding: 0;}
        
        .fbox{
            width: 1500px;
            height: 300px;
            border: 1px solid #000;
        }
        
        .sbox1{
            width: 200px;
            /* 因为box2的after清楚了浮动，使得box2的高度适中跟随box1的高度变化 */
            height: 700px;
            float: left;
            background: rebeccapurple;
            opacity: .5;
        }
        .ssbox{
            width: 50px;
            height: 50px;
            border: 1px solid #000;
            background: gray;
            float: left;
            
        }
        .sbox2{
            width: 300px;
            background: yellow;
            margin-left: 300px;
        }
        .sbox2::after{
            content: "";
            display: block;
            clear: both;
        }
    </style>
</head>
<body>
    <div class="fbox">
        <div class="sbox1">BOX1</div>
        <div class="sbox2">BOX2
            <div class="ssbox"></div>
            <div class="ssbox"></div>
            <div class="ssbox"></div>
        </div>
    </div>   
</body>
</html>